Effect of barrier layer thickness and composition on fracture toughness of layered zirconia/alumina composites

Composites of yttria or ceria-partially-stabilized zirconia with layers of either alumina or a mixture of 50% by volume of alumina and zirconia were fabricated by sequential centrifuging of powder suspensions. This method allowed formation of layers with thickness of 10 to 70 m. In both cases (Y-ZrO₂ and Ce-ZrO₂ matrices), a significant increase in fracture toughness, work of fracture and bending strength was observed only for composites with barrier layers made of a pure alumina. A crack deflection in alumina layer was found to be the main mechanism responsible for an increase in mechanical properties. For confirmation this thesis, no increase in the transformation zone width was observed. As it was shown, crack deflection angle was dependent on alumina layer thickness. Higher deflection angles for a thicker alumina layers were found. Explanation of this phenomenon was given by determination of residual stress distribution in barrier layers made by piezospectroscopy. A correlation between the crack deflection angle and the difference of stress between the layer boundary and the centre of the layer was noticed. The residual stresses observed are a result of thermal expansion mismatch between alumina and zirconia and thermal anisotropy of alumina. Shrinkage mismatch, especially in the case of Ce-ZrO₂ and Al₂O₃, as a third source of stress is suggested.